The present invention relates generally to vascular imaging systems, and in particular to intravascular ultrasound image production devices and data processing methods that enable the user to visualize blood flow in intravascular ultrasound (IVUS) images.
IVUS imaging is widely used in interventional cardiology as a diagnostic tool to establish the need for treatment of a diseased artery, to determine the most appropriate course of treatment, and to assess the effectiveness of the treatment. IVUS imaging uses ultrasound echoes produced by a catheter having an ultrasound-producing transducer to form a cross-sectional image of a tubular structure such as, but not by way of limitation, a vessel of interest. Typically, the transducer both emits ultrasound signals and receives reflected ultrasound echoes. The catheter is placed in the vessel such that the transducer is located at a region of interest in the vessel. The ultrasound waves pass easily through most tissues and blood, but they are partially reflected from discontinuities arising from red blood cells, tissue structures (such as the various layers of the vessel wall), and other features of interest. The IVUS imaging system processes the received ultrasound echoes to produce a two-dimensional, cross-sectional image of the vessel in the region of the transducer.
To establish the need for treatment, the IVUS system is used to measure the lumen diameter or cross-sectional area of the vessel. For this purpose, it is important to distinguish blood from vessel wall tissue so that the luminal border can be accurately identified. In an IVUS image, the blood echoes are distinguished from tissue echoes by slight differences in the strengths of the echoes (e.g., vessel wall echoes are generally stronger than blood echoes) from subtle differences in the texture of the image (i.e., speckle) arising from structural differences between blood and vessel wall tissue and relative motion across frames.
As IVUS imaging has evolved, there has been a steady migration towards higher ultrasound frequencies to improve the resolution in the display. But as ultrasound frequency is increased, there is diminished contrast between the blood echoes and vessel wall tissue echoes. At the 20 MHz center frequency used in early generations of IVUS, the blood echoes are very weak in comparison to the vessel wall echoes due to the small size of the red blood cell compared to the acoustic wavelength. However, at the 40 MHz ultrasound center frequency now commonly used for IVUS imaging, there is only a modest difference between blood and tissue echoes because the ultrasound wavelength at this higher frequency is closer to the dimensions of the red blood cells.
Another use of IVUS imaging in interventional cardiology is to help identify the most appropriate course of treatment. For example, IVUS imaging may be used to assist in recognizing the presence of thrombi (e.g., coagulated blood that is stationary within the blood vessel, such as, for example, mural thrombi) in an artery prior to initiating treatment. If a thrombus is identified in a region where disease has caused a localized narrowing of the arterial lumen, then the treatment plan could be modified to include aspiration (i.e., removal) of the thrombus prior to placing a stent in the artery to expand and stabilize the cross-sectional area of the vessel. In addition, the identification of a thrombus could trigger the physician to order a more aggressive course of anti-coagulant drug therapy to prevent the subsequent reoccurrence of potentially deadly thrombosis. In a conventional IVUS image, however, there is very little difference in appearance between thrombi and moving blood.
Yet another use of IVUS imaging in interventional cardiology is to visualize the proper deployment of a stent within an artery. A stent is an expandable cylinder that is generally expanded within the artery to enlarge and/or stabilize the lumen of the artery. The expansion of the stent often stretches the vessel and displaces the plaque formation that forms a partial obstruction of the vessel lumen. The expanded stent forms a scaffold propping the vessel lumen open and preventing elastic recoil of the vessel wall after it has been stretched. In this context, it is important to recognize proper stent apposition; that is, the stent struts should be pressed firmly against the vessel wall. A poorly deployed stent may leave stent struts in the stream of the blood flow and these exposed stent struts are prone to initiate thrombus formation.
Thrombus formation following stent deployment is referred to as “late stent thrombosis” and these thrombi can occlude the artery or break free from the stent strut to occlude a downstream branch of a coronary artery and trigger a heart attack.
In these examples of intravascular IVUS imaging, it is particularly useful to identify moving blood and to distinguish the moving or dynamic blood from relatively stationary or static tissue or thrombi. Motion information can be helpful in delineating the interface between blood and vessel wall so that the luminal boundary can be more easily and accurately measured. Motion parameters such as velocity may be the most robust ultrasound-detectable parameters for distinguishing moving blood from stationary thrombi. For example, in the case of stent malapposition, the observation of moving blood behind a stent strut is a clear indication that the stent strut is not firmly pressed against the vessel wall as it should be, and may indicate a need to redeploy the stent. In each of the aforementioned uses of IVUS, the addition of motion parameters to the traditional IVUS display of echo amplitude can improve the diagnosis and treatment of a patient.
Traditionally, IVUS catheters, whether rotational or solid-state catheters, are side-looking devices, wherein the ultrasound pulses are transmitted substantially perpendicular to the axis of the catheter to produce a cross-sectional image representing a slice through the blood vessel. The blood flow in the vessel is normally parallel to the axis of the catheter and perpendicular to the plane of the image. IVUS images are typically presented in a grey-scale format, with strong reflectors (vessel boundary, calcified tissue, metal stents, etc.) displayed as bright (white) pixels, with weaker echoes (blood and soft tissue) displayed as dark (grey or black) pixels. Thus, flowing blood and static blood (i.e., thrombi) may appear very similar in a traditional IVUS display.
In other (e.g., non-invasive) ultrasound imaging applications, Doppler ultrasound methods are used to measure blood and tissue velocity, and the velocity information is used to distinguish moving blood echoes from stationary tissue echoes. Commonly, the velocity information is used to colorize the grey-scale ultrasound image in a process called color flow ultrasound imaging, with fast moving blood tinted red or blue, depending on its direction of flow, and with stationary tissue displayed in grey-scale.
Traditionally, IVUS imaging has not been amenable to color flow imaging because the direction of blood flow is predominantly perpendicular to the IVUS imaging plane. More specifically, Doppler color flow imaging and other Doppler techniques do not function well when the velocity of interest (i.e., blood flow velocity) is perpendicular to the imaging plane and perpendicular to the direction of ultrasound propagation, thereby causing almost zero Doppler shift attributable to blood flow. In the case of rotational IVUS, there is an added complication due to the continuous rotation of the transducer, which makes it problematic to collect the multiple echo signals from the same volume of tissue needed to make an accurate estimate of the velocity-induced Doppler shift. Various image correlation methods attempt to overcome the directional limitations of the Doppler method for intravascular motion detection, but are generally inferior to Doppler methods. Moreover, such image correlation techniques are not suitable for rotational IVUS because the rate of decorrelation due to the rotating ultrasound beam is comparable to the rate of decorrelation for the blood flow.
Accordingly, there is a need for apparatuses, systems, and/or methods that can produce intravascular images that better differentiate between dynamic and static contents within a vessel. The methods disclosed herein overcome one or more of the deficiencies of the prior art.